Johnson has been less than sympathetic to the Irish govt position that there can be no hard border. Now, if he challenges May with a "no confidence" vote he would well replace her which would throw the entire border issue up into the air.

The Johnson resignation also has major implications for the DUP, the main unionist party which has been propping up May in power, as it could trigger a general election which would likely see the DUP lose their influence on a new government.